The voltages are stored in the raw disk. They're called the waveform data.
Kobo provides a way to update this waveform data as part of software updates but, sadly, I couldn't find that specific blob in any fw upgrade.
A few years ago I repurposed some "broken" kobos for a local library and I remember I had to dump and copy data between devices to have the proper settings when swapping screens between them.
I'm pretty bad taking notes but a quick google search returned this
https://news.ycombinator.com/item?id=14085861
The blob is loaded by uboot. I could provide more info if you are interested